Mrs Clinton received the infected emails, disguised as speeding tickets, over four hours early one morning in August 2011. The emails instructed recipients to print the attached tickets, which would have allowed hackers to take control of their computers.

Security researchers who analysed the malicious software have said that infected computers would transmit information from victims to at least three server computers overseas, including one in Russia. That does not necessarily mean Russians were responsible.

Nick Merrill, a spokesman for Clinton’s Democratic presidential campaign, said: “We have no evidence to suggest she replied to this email or that she opened the attachment. As we have said before, there is no evidence that the system was ever breached. All these emails show is that, like millions of other Americans, she received spam.”